DE60038355T2 - Process for producing ceramic coatings - Google Patents

Description

These The invention relates to a process for the production of ceramic Thermal barrier coatings and abradable gaskets consisting of several layers, wherein at least one of the layers is porous.

Gas turbine engines are widely used as propulsion sources and for other purposes as for electricity generation and used for pumping fluids. Manufacturer of gas turbines are with a constant customer demand for better performance, increased efficiency and improved service life. One way, the performance and the performance To improve is to increase the operating temperatures. One Increase The operating temperatures usually reduce the service life the machine and is only within the limits of the machine used materials effectively.

Current gas turbine engines are mainly constructed of metallic materials, in the areas of the machine higher Temperature superalloys based on nickel and cobalt in wide Scope be used. Such superalloys are currently being included Gas temperatures very close to, and in some cases over, the Melting point of superalloys are used in machines. increases the machine operating temperature is not possible without simultaneous steps, to protect the superalloys from these high gas temperatures which would otherwise melt the materials directly exposed to become. Such steps include the provision of cooling air (which reduces the machine efficiency) and the use of insulating Coatings.

Insulating ceramic materials, particularly the provision of these materials in the form of coatings or thermal barrier coatings, are the main subject of this invention. Such coatings are most commonly ceramic and are commonly applied by plasma spraying or electron beam vapor deposition. Electron-beam vapor deposited coatings are described, for example, in U.S. Pat U.S. Patents 4,405,659 ; 4,676,994 and 5 087 477 , Exemplary patents discussing the current state of the art thermal barrier coatings include US Pat U.S. Patent Nos. 4,321,311 ; 4 405 660 . 5,262,245 and 5 514 482 ,

The most widely used thermal barrier coating for application to rotating components in turbine engines has a bond coat material whose composition in the U.S. Patent No. 4,419,416 including a thin layer of alumina on the bond coat and a columnar grain ceramic coating adhered to the alumina layer, as described in U.S. Pat U.S. Patent No. 4,405,659 , Despite the success of this thermal barrier coating and its widespread acceptance, there is a desire for more advanced thermal barrier coatings where the desired major improvement is improved specific thermal insulation properties, ie density enhanced thermal insulation.

If improved a coating with better density Insulating properties could be developed, could such a coating either at the same thickness as the one now used to reduce the Heat flow commercial used, which allow a reduction in cooling air and allow a corresponding increase in engine efficiency would, or she could be used at a reduced thickness to the same degree insulation and heat flow, but with a reduced coating weight. such Weight reductions are significant, especially with rotating ones Components, as the weight of the thermal barrier coating while of machine operation to centrifugal forces of thousands of pounds on a single turbine blade in a large aircraft engine leads. A reduction in blade centrifugal forces has a positive effect on the design requirements associated with the blade Machine components, in particular the support disk.

Gas turbine efficiency can also be improved by reducing gas leaks. In particular, the free space between the ends of the rotating blade and the surrounding housing structure must be minimized. This is usually accomplished by providing an abradable sealing material on the housing. In operation, the blade ends cut a channel into the abradable material, which reduces the leakage of gas. See, for example, B. the U.S. Patents No. 4,039,296 and no. 5 536 022 ,

EP-A-0916635 and US-A-5,350,599 relate to a method as indicated in the preamble of claim 1.

The The present invention provides a method as claimed in claim 1 is ready, ready.

The present invention therefore includes a method of applying a layered ceramic material coating. Different layers in the structure have different micro structures, wherein at least one of the layers is relatively dense and contains fewer defects, and another of the layers is less dense and contains more defects. The structure of the less dense layer can be modified by a heat treatment to provide porosity. Porosity provides reduced thermal conductivity, and for seal applications, porosity also provides improved abradability. The layers are preferably deposited by electron beam physical vapor deposition. The layers are applied under conditions that produce the aforementioned differences in density and porosity between alternating layers by changing the temperature of the substrate and the material as deposited.

The relatively dense layers are formed by electron beam vapor deposition under conditions that lead to the separation of what those with knowledge in the field of physical vapor deposition as Zone II structures denote, applied. The less dense layers (i.e. Layers that are porous are also applied by EBPVD and under conditions that lead to microstructures, the experts in the field of physical vapor deposition denote zone I structures. The term zone I, as used herein a layer that is either a zone (I-structure, as deposited, or a zone I structure, such as deposited, to increase the porosity heat treated was, has.

The The resulting structure can be heat treated be to the porosity by sintering, which increases the pore size and the ceramic regions, which surround the pores, compacted.

The Coating of the invention finds particular application to the Field of gas turbine components. Such components include turbine airfoils (Blades and vanes) and Abreibbare seals that are meant to be, with blade ends or cutting gaskets to interact to an undesirable gas flow to reduce.

Now certain preferred embodiments in more detail, by way of example only and with reference to FIGS accompanying drawings, in which:

1 shows the structure of EB-PVD deposited coatings as a function of substrate surface temperature;

2 Figure 3 shows the microstructure of a coating applied as deposited in accordance with a preferred embodiment of the invention;

3 shows the microstructure of another coating applied after a heat treatment according to a preferred embodiment of the invention; and

4 the thermal conductivity of an exemplary coating of the invention and a corresponding coating that lacks the layered porosity provided by the present invention.

Physical vapor deposition of ceramics, including electron beam and sputtering techniques, has been extensively studied. A work published in 1969 by BA Movchan and AB Demchishin in the Journal of Physics of Metallurgy and Metallography (USSR), Vol. 28, p. 83, entitled "Study of the Structure and Properties of Thick Vacuum Condensates of Nickel , Titanium, Tungsten, Aluminum Oxide, and Zirconium Oxide "analyzes the vapor deposition of materials under various conditions. These authors were the first to characterize the structure of electron beam vapor deposited coatings as a function of substrate surface temperature. 1 the work is referred to herein as 1 played. In 1974, Thornton published a similar work on sputter deposition, J.Vac. Science Technology 11: 666.70 (1974), with similar conclusions.

1 Figure 3 shows the three zones that occur in physical vapor deposited or physically vapor deposited coatings as a function of substrate surface temperature during deposition. Zone I is a low density coating with a high defect content, which may contain micropores, micro voids, dislocations, voids and the like. Zone II is a denser columnar grain structure in each column, and is a single grain. Zone III is an even denser, substantially completely dense coating composed of equiaxed recrystallized grains. It is believed, although not fully understood, that the change in deposition characteristic that actually increases with increasing substrate temperature density results from the increased mobility of vapor deposited atoms after their impact on the substrate surface.

For ceramics, Movchan et al. Determined that the boundary between Zone I and Zone II typically occurred at a homologous temperature (homologous temperature refers to the fraction of the absolute melting temperature of the material) of between 0.22 and 0.26, and the limit between zones II and III typically at a homologous temperature of between about 0.45 and 0.5 occurred.

It It can be seen that a single ceramic composition so It can be separated that they have three different structures has, by looking at the temperature of the substrate surface on which the coating is deposited, controlled. Because the microstructure of the deposited Coating is a function of the substrate temperature, you can also see that different ceramic materials with different Melting points and therefore different homologous temperatures deposited on a substrate at constant surface temperature could become and could have different zone structures.

The Substrate surface temperature refers to the temperature at the surface on which the coating is separated, this temperature is often different from the Temperature of the substrate bulk and is due to radiant energy flow and the energy input into the surface by coating material, that hits the surface, influenced and can by the use of external heating devices be increased. higher Substrate surface temperatures allow lateral diffusion of deposited atoms, what to higher deposits Density leads.

Of the The core of the present invention is a method for deposition at least one layer with a zone I structure and at least a layer with a zone II or zone III structure. The deposition can be a suitable heat treatment consequences. A guideline for a suitable heat treatment temperature is a temperature above about 0.5 to 0.8 of the homologous temperature of the layer composition, about the porosity to accomplish. Preferably, there is a layer of the zone I structure between adjacent layers of Zone II structure. The zone I structure, the less dense is, inhibits the heat flow and therefore provides for a thermal insulation. The less dense structure is also abradable for sealing applications. The Layer with the relatively denser Zone II structure ensures mechanical Strength. The layer thickness can vary, for example between 0.05 and 5000 μm, and preferably between 0.1 and 1000 μm. The total coating thickness For example, it can range from about 10 μm to about 1 mm for thermal barrier coatings and up to about 5 mm for abradable coatings lie.

As at the above mentioned Application discussed practical applications of the invention many alternating layers of materials, e.g. Eg more than 10 layers and even more than 100 layers, included. Put practical applications of the invention also prefers heat treatments after depositing the layered coating, insert one Sintering of the Zone I structure and the formation of larger, accumulated pores or porosity to induce. Such a pore accumulation improves the mechanical Properties of the coating by compacting the ceramic in areas between the pores. The average pore size exceeds 0.01 and preferably 0.1 microns. The resulting pores have a rounded shape through them that is characterized for a Pore of average diameter "D" (measured at several diameters) the smallest radius of the pore wall is greater than 0.1 D, and preferably greater than 0.3 D, is.

Usually is a bonding order (or at least an alumina layer) between the substrate and the coated porosity coating of the invention appropriate. Preferred binding jobs include those coatings, known as MCrAlY coatings and aluminide coatings are. Both types of coatings form dense, adherent alumina layers of more reasonable Purity, and it is this alumina layer to which the coating of the invention liable.

The Concept of this invention can be appreciated by considering the following example, that is illustrative, not limiting, better be understood.

example

A single crystal superalloy substrate having a nominal composition of 5% Cr, 10% Co, 1.9% Mo, 5.9% W, 3% R, 8.7% Ta, 5.65% Al, 0.1 % Hf, balance Ni was provided. The surface of the substrate was cleaned by blasting with alumina to form a thin MCrAlY-type bond coat layer having a nominal thickness of about 125 μm (0.005 inches) and a nominal composition of about 22% Co, 17% Cr, 12.5 % Al, 0.25% Hf, 0.4% Si, 0.6% Y, balance Ni. The binding deposit was deposited by conventional plasma spraying techniques. The bond coat surface was then hammered with glass beads to increase their density. Then, a heat treatment step was performed to develop a thermally grown oxide layer (predominantly alumina). The treatment was carried out at 816 ° C (1500 ° F) for 15 minutes at an oxygen flow of 70 standard cubic centimeters per minute (sccm) at a pressure of about 3.3 mPa (10 ^-4 torr). Minimal experimentation is required to develop the desired oxide thickness.

Then a layered coating was applied to the bond coat surface, composed of alternating layers of yttria-stabilized ceria (12 wt.% yttria) alternately between less dense and denser layers. The coating thicknesses were about 0.25 μm per layer. Several pairs of coating layers were applied. See, for example, B. 2 ,

These Coating layers were made using an electron beam applied to the evaporation of the ceramic materials. The source material was by means of an electron beam at about 10 kV and a Current of about 0.4 A for an evaporation power of about 4 kW worked, evaporated. Of the Distance from the substrate to the ceramic source was about 76 mm (3 inch). The substrate was during turned to the Abscheiung.

The beam lingered continuously on the source, with a metal thermal barrier shield applied repeatedly for 16 second intervals between the source and the substrate, and then followed by a gap for 8 second intervals. The shield was made of stainless steel and contained openings of about 4 mm. The coatings were applied at a reduced pressure of about 532 x 10 ^-6 Pa (4 x 10 ^-6 torr) and oxygen was flowed into the room at a rate of about 70 sccm to ensure coating stoichiometry.

The thermal radiation from the target evaporating material was the major source of heating of the substrate surface (above the temperature of the bulk substrate) during the electron beam physical vapor deposition process. The substrate surface was heated to a greater extent in the absence of the shield attached between the source and the substrate, and to a lesser extent when the shield was mounted between the source and the substrate to reduce heat flow from the source to the substrate. This process resulted in alternating yttria-ceria layers having a denser Zone II structure and layers having a less dense Zone I structure containing micro-voids and microporosity. See, for example, B. 2 , The estimated porosity of the zone II yttria ceria, as deposited, was relatively low, e.g. A few percent, and the porosity was significantly higher (a porosity of about 50%) in the zone I-yttria-ceria.

The Yttria-ceria composition has a relatively high vapor pressure and evaporates easily at a much lower temperature, obviously by sublimation, and therefore does not form a hot molten pool like that while the real evaporation formed. Materials such as yttria-ceria, which have a relatively high vapor pressure, deliver during the Evaporation less heat as other materials, such as yttria-stabilized zirconia, and warm up therefore, the substrate to a lesser extent. In these cases can it would be desirable use external heaters to increase the substrate temperature to increase. In that case, would the shield between the source and the substrate, between the external heater and the substrate, or both become.

All in all was the substrate surface, which has been coated while the deposition of yttria-ceria in the absence of the shield at a relatively high temperature, and the vapor condensed as a structure of Zone II type. The substrate surface was however during the Deposition of yttria-ceria, at between the source (and any external heater) and the substrate Shielding, and limited heating, at a relatively low Temperature, and the associated vapor cloud condensed as a zone I type structure.

In the case of yttria-ceria or other material that may not adhere well to a bond coat or alumina layer when deposited at lower temperatures (another reason for using an external heater above), it may be desirable be to use a layer of a ceramic bond coat material, such as yttria-stabilized zirconia, as an initial layer or ceramic "bond coat". Such a ceramic bonding order is in EP-A-0 972 853 described. The layer should be thick enough to ensure complete coverage of the surface to be coated up to, for example, about 12.7 μm (0.5 mil) but not thicker than necessary to ensure a continuous layer, especially when weight is an issue is, for example, when the coating is to be applied to rotating components.

Other samples were prepared using gadolinia stabilized zirconia. The samples contained about 33 moles. % Gadolinia, balance 7YSZ as the stabilized zirconia, and were deposited in a similar manner to that described above. The coating, as deposited, was then heat treated at about 1205 ° C (2200 ° F) for about 24 hours. The resulting layered porosity is in the range of 10 indicated by 3 illustrated.

4 FIG. 12 is a graphical representation of the thermal conductivity coating of a preferred embodiment of the present invention as described. FIG 2 described, and a corresponding coating with non-layered porosity. It can be seen that over a wide range of temperatures the thermal conductivity of the coating of the invention is substantially lower than that of the prior art coating. Over the temperature range examined, the coating of the present invention exhibits a thermal conductivity significantly lower than that of the prior art coating. This represents a significant improvement in insulation ability.

The previous examples illustrate the invention using a composition of ceramic material. There are of course many compositions and combinations of ceramic materials that are deposited can. Some Variations and alternative details are described below.

substratum

In most widely, the substrate may be any high temperature material such as, ceramics, carbon, carbon composites and the like, as well as superalloys. For the purpose current Gas turbine engines are superalloys most useful. superalloys are metallic materials based on iron, nickel or Cobalt with yield strengths at 538 ° C (1000 ° F) over 345 MPa (50 ksi), and more typically about 690 MPa (100 ksi).

Bonding applications, as discussed below, are often desirable to ensure adhesion of the coating to the substrate, but under some conditions, there may be no need for a bond application for some superalloys. Superalloys that can be coated without the use of a bond can inherently, when exposed to oxidizing conditions at elevated temperatures, develop a layer of high purity aluminum on their exterior surfaces. An exemplary superalloy that does not require bonding is in the US patent application Ser U.S. Patent No. 5,262,245 described.

If the coating of the invention on a ceramic material, for example Alumina, to apply, there would be generally no need for a binding job.

binding contract

Usually is a bond coat part of the coating system of the invention. The requirement of a bond coat depends on the substrate and from the requirements of the performance of the thermal barrier coating, the Operating temperature, the desired Lifetime and other environmental factors including cyclic thermal stress and the ambient gas composition. The essential common Features required for a usable binding job are, that it adheres to the substrate, thermally and diffusion moderately stable is, and forms a stable alumina layer in the operating environment adheres to the deposited ceramic layer.

There are a variety of binding jobs that can be used in the present invention. These include overlay coatings and aluminide coatings. Typical overlay coatings are in the U.S. Patent No. 3,928,026 and no. 4 419 416 and are modifications of nickel- and cobalt-based superalloys that have been optimized to develop adherent, high-purity, high-purity alumina layers when exposed to oxidizing conditions at elevated temperatures. Pad coatings are applied to the surface of the substrate, and typical of them are the MCrAlY-type coatings, which have the following general non-limiting composition (in weight percent): Cr 10-30, Al 5-15, Y (Y + Hf + La + Ce + Sc) 0.01, Si + Ta + Pt + Pd + Re + Rh + Os 0-5%, balance M (Fe, Ni, Co and combinations). Pad coatings can be applied by, for example, plasma spraying, EB-PVD and electroplating.

Aluminide coatings are made by diffusing aluminum into the substrate and are, for example, in the U.S. Patent No. 5,514,482 described. The term aluminide coating as used herein includes aluminide coatings modified by additions of Pt, Rh, Os, Pd, Ta, Re, Hf, Si, Cr, and mixtures thereof.

It is also known combinations of overlay and aluminide coatings For example, an aluminide coating may be one above the Aluminide applied bearing coating and vice versa.

Alumina layer

An alumina layer, whether formed directly on the substrate or on a bond coat, is an important feature of the invention. Usually, the alumina layer is allowed to grow thermally, but it is known to use sputtering to develop this layer. The broad thickness of the alumina layer is about 0.01 to 2 μm, preferably about 0.1 to 0.7 μm.

Ceramic compositions

Although yttria-ceria and gadolinia-zirconia are described in the above examples, practically any ceramic composition which can be applied by physical vapor deposition can be used in the present invention. Other suitable ceramics, including gadolinia-zirconia, are disclosed in U.S. Pat EP-A-0 992 603 and EP-A-0 848 077 disclosed.

The Ceramics should work with the bond coat and / or substrate, the other ceramic compositions present in the coating, and the working environment be. The ceramic coating can be a single ceramic composition (as above) or various ceramic compositions known as alternating layers are applied. The critical The characteristic is that the coating is treated in such a way that it alternates Zone I / Zone II (or Zone III) layer structures generated. Prefers The ceramics used are those that are intrinsically low Mass Thermal Conductivity exhibit. This includes stabilized zirconia, ceria with rare earth additives and Pyrochloroxid connections.

The Arrangement of the ceramic coating can be quite varied. The broadest description is that the coating consists of several layers of which at least one is deposited so that they has a structure of zone I type. The zone I structure is preferred heat-treated afterwards, to porosity to form or to increase. The layer of the zone I structure also preferably has an immediate effect they adjacent layers with zone II (and / or zone III) structure, around for a mechanical support and limit. Preferably, there are a plurality of deposited layers of zone I type, that of layers with structures of zone II / zone III type to be separated.

We believe that fine porosity is most effective in reducing the heat flow. This must be against Be weighed the thought that small pores less stable are because they tend to shrink at high temperatures and to shut up or heal. Pore closure is not a significant problem when the use temperature is lower than the pore shrinkage temperature.

Under some circumstances can other layers are incorporated without the benefits of thermal insulation to affect the invention. For example can be an outer layer chosen be in order for to provide certain properties that are desirable for certain applications, such as heat radiating ability, hardness, abrasion resistance, durability against environmental attack (oxidation, sulfidation, nitridation, etc.) and / or resistance to Diffusion of harmful Environmental species such as oxygen, the underlying coating layer and / or adversely affect the binding application and / or the substrate would. For example, could Alumina as an outer layer he wishes because it is relatively hard and against the diffusion of oxygen resistant is.

For sealing applications meet many of the same considerations to. Overall coating thickness is generally greater until about 2.54 mm (100 mils), preferably 1.27 mm (50 mils). The single layer thickness and the pore volume and pore size are optimized to the for one Abreibbarkeit required mechanical properties provide.

Claims (14)

A method of applying a layered ceramic coating to a substrate, comprising: vapor depositing a plurality of ceramic layers from at least one source of evaporant material and onto the substrate, and changing the temperature during vapor deposition to form alternating layers of ceramic material, wherein at least one layer is at one depositing at least one other layer at a second temperature other than the first temperature, at least one of the layers having a Zone I microstructure having a relatively less dense structure, and at least one of the layers forming a zone II microstructure, which has a denser structure than the zone I structure, wherein the layer with zone II microstructure is deposited at a higher temperature than the layer with the zone I microstructure, and characterized in that the step of Changing the temperature by periodic A a heat-insulating shield between the evaporation material source and the substrate in order to reduce the heat flow, is performed. A method as claimed in claim 1, wherein a single source of evaporative material is used. A method as claimed in claim 2, wherein the only source of evaporation material from a stabilized ceria is composed. A method as claimed in claim 2, wherein the sole source of evaporation material is one of stabilized zirconia is composed. A method as in any preceding claim in which the substrate has a surface layer, mainly alumina has, has. A method as claimed in claim 5, wherein a binding job that is selected is made up of the group consisting of MCrAlY and aluminide layers and combinations of which is applied to the substrate and the alumina layer on the surface the binding job is deposited. A method as claimed in claim 5, wherein The substrate is made of a material that forms an adhesive Aluminum oxide layer is able to exist, and that also the Step of forming an alumina layer on the substrate having. A method as claimed in claim 5, 6 or 7, wherein the alumina layer has a thickness of 0.1 to 2.0 μm. A method as in any preceding claim claimed in that the total thickness of the layers from 0.05 to 5,000 μm is. A method as in claim 1, wherein the step vapor deposition, vaporizing at least two sources of ceramic material includes. A method as in claim 10, wherein the ceramic material sources have different compositions. Method as in any preceding claim, wherein the substrate defines a gas turbine engine component. Method as in any preceding claim, wherein the substrate is made of a superalloy material. A method as in any preceding claim claimed, as well comprising the step of heat treating the layer to be in one or more of the layers with a zone I microstructure porosity to create.